Featured image (Thumbnail) Display large on all my post.

Please i need help, i generate thumbnail for all my post so it can display in my Related posts: but when i opened any post in my website the thumbnail for that post will display very large on top of that post.
please i don’t know if the problem is from any plugin, the theme or the WordPress version 3.7. I cant just change my theme, that is a long way to go
site somuchworld.com